In the given figure, two wheels $P$ and $Q$ are connected by a belt $B$. The radius of $P$ is three times that of $Q$. In the case of the same rotational kinetic energy, the ratio of rotational inertias $\left( \frac { I _ { 1 } } { I _ { 2 } } \right)$ will be $x : 1$. The value of $x$ will be $\_\_\_\_$.